Dual container

ABSTRACT

The present invention is a dual container including an inner container deformed to reduce a volume according to reduction of accommodated contents, and an outer container in which the inner container is inserted and which is elastically deformable. In the dual container, a mouth portion, a shoulder portion, a drum portion, and a bottom portion are arranged in sequence from top to bottom in a bottle axis direction, an outside air introduction hole configured to introduce outside air between the outer container and the inner container according to reduction of contents is provided, a circumferential groove recessed inward in a radial direction and extending in a circumferential direction is formed in the drum portion, and a longitudinal convex portion configured to disconnect the circumferential groove in the circumferential direction is provided on an inner surface of the circumferential groove.

CROSS-REFERENCE TO RELATED APPLICATION

Priority is claimed on Japanese Patent Application No. 2021-160790, filed Sep. 30, 2021, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a dual container.

Description of Related Art

In the related art, for example, as disclosed in Japanese Unexamined Patent Application, First Publication No. 2019-131256, a dual container is known. The dual container includes an inner container deformed to reduce its volume according to reduction in accommodated contents and an outer container in which the inner container is inserted and which is elastically deformable. In the dual container, a mouth portion, a shoulder portion, a drum portion, and a bottom portion are arranged in sequence from top to bottom in a bottle axis direction, and an outside air introduction hole configured to introduce outside air is provided between the outer container and the inner container according to reduction in contents.

However, in the dual container in the related art, there is room for improvement in increasing a restoring force of the outer container without impeding smooth peeling of the inner container from an inner surface of the outer container.

In consideration of the above-mentioned circumstances, the present invention is directed to providing a dual container capable of increasing a restoring force of an outer container without impeding smooth peeling of an inner container from an inner surface of the outer container.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a dual container including: an inner container deformed to reduce a volume according to reduction of accommodated contents; and an outer container in which the inner container is inserted and which is elastically deformable. In the dual container, a mouth portion, a shoulder portion, a drum portion, and a bottom portion are arranged in sequence from top to bottom in a bottle axis direction, an outside air introduction hole configured to introduce outside air between the outer container and the inner container according to reduction of contents is provided, a circumferential groove recessed inward in a radial direction and extending in a circumferential direction is formed in the drum portion, and a longitudinal convex portion configured to disconnect the circumferential groove in the circumferential direction is provided on an inner surface of the circumferential groove.

The circumferential groove is formed in the drum portion. Accordingly, rigidity of the drum portion of the outer container can be increased, and after the drum portion is squeezed inward in the radial direction by applying an external force to the drum portion, the drum portion of the outer container can be smoothly restored when the external force (i.e., pressing force) is released.

The longitudinal convex portion configured to disconnect the circumferential groove in the circumferential direction is formed on the inner surface of the circumferential groove. Accordingly, the outside air introduced between the inner container and the outer container from the outside air introduction hole can easily pass through the longitudinal convex portion in the upward/downward direction, and the inner container can easily peel off from the inner surface of the outer container in the circumferential groove from the longitudinal convex portion as an origin. Accordingly, the inner container including the portion in which the circumferential groove is formed can smoothly peel off from the inner surface of the outer container according to reduction of the accommodated contents.

A second aspect of the present invention is the dual container according to the above-mentioned first aspect. In the dual container, an outer surface of the longitudinal convex portion, the outer surface facing outward in the radial direction, is continuous with an outer circumferential surface of each portion of the dual container, the portion being continuous with the circumferential groove in the upward/downward direction, with no step difference in the upward/downward direction.

The outer surface of the longitudinal convex portion facing outward in the radial direction is continuous with the outer circumferential surface of each portion continuous with the circumferential groove of the dual container in the upward/downward direction with no step difference in the upward/downward direction. Accordingly, the outside air introduced between the inner container and the outer container from the outside air introduction hole can reliably pass through the longitudinal convex portion in the upward/downward direction.

A third aspect of the present invention is the dual container according to the above-mentioned first or second aspect. In the dual container, at least a lower portion of the drum portion extends further outward in the radial direction as it goes downward, and the circumferential groove is provided in the lower portion of the drum portion.

At least the lower portion of the drum portion extends outward in the radial direction as it goes downward, and the circumferential groove is provided in the lower portion where it is difficult for the outer container to be restored after squeeze deformation. Accordingly, rigidity of the lower portion of the drum portion of the outer container can be increased, and the lower portion of the drum portion of the outer container can be easily restored after squeeze deformation.

Since the longitudinal convex portion is formed on the inner surface of the circumferential groove, smooth peeling of the inner container from the inner surface of the outer container in the lower portion of the drum portion can be secured.

According to the present invention, it is possible to increase a restoring force of the outer container without impeding smooth peeling of the inner container from the inner surface of the outer container.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view including a partial longitudinal cross section when a dual container shown as an embodiment is seen from an outer side in a radial direction.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a dual container according to an embodiment will be described with reference to the accompanying drawings.

As shown in FIG. 1, a dual container 1 according to the embodiment includes an inner container 12 deformed to reduce a volume according to reduction in accommodated contents, and an outer container 11 in which the inner container 12 is inserted and which is elastically deformable. An outer surface of the inner container 12 is provided on an inner surface of the outer container 11 to be separable therefrom. In an example shown, the inner container 12 is highly flexible and is stacked on the inner surface of the outer container 11 so that it can be peeled off.

Further, a gap may be provided between the inner surface of the outer container 11 and the outer surface of the inner container 12.

The dual container 1 is formed by integrally blow-molding an outer preform and an inner preform in a state in which the inner preform configured to form the inner container 12 is fitted into the outer preform configured to form the outer container 11. That is, the dual container 1 is a biaxial stretching blow container.

Materials of the inner container 12 and the outer container 11 are synthetic resin materials, which may be the same material or may be different materials. As an example of the synthetic resin material, for example, polyethylene terephthalate (i.e., PET), polypropylene (i.e., PP), polyethylene (i.e., PE), nylon (i.e., polyamide), and ethylene-vinyl alcohol copolymer (i.e., EVOH), or the like, may be exemplified.

In the example shown, each of the inner container 12 and the outer container 11 is formed of polyethylene terephthalate (i.e., PET).

The dual container 1 includes a mouth portion 13, a shoulder portion 15, a drum portion 16 and a bottom portion 14. The mouth portion 13, the shoulder portion 15, the drum portion 16 and the bottom portion 14 are disposed coaxially with a common axis in sequence.

Hereinafter, the common axis is referred to as a bottle axis O, a side of the mouth portion 13 of the dual container 1 along the bottle axis O is referred to as an upper side, and a side of the bottom portion 14 of the dual container 1 is referred to as a lower side. A direction crossing the bottle axis O when seen in a bottle axis O direction (i.e., an upward/downward direction) is referred to as a radial direction, and a direction around the bottle axis O is referred to as a circumferential direction.

The mouth portion 13 of the dual container 1 is configured by stacking a mouth portion of the inner container 12 and a mouth portion of the outer container 11. The shoulder portion 15 of the dual container 1 is configured by stacking a shoulder portion of the inner container 12 and a shoulder portion of the outer container 11. The drum portion 16 of the dual container 1 is configured by stacking a drum portion of the inner container 12 and a drum portion of the outer container 11. The bottom portion 14 of the dual container 1 is configured by stacking a bottom portion of the inner container 12 and a bottom portion of the outer container 11.

In the following description, both the inner container 12 and the outer container 11 have the same form unless the context clearly indicates otherwise.

A size of the dual container 1 in the bottle axis O direction is, for example, 115 mm or more and 220 mm or less. An inner capacity of the dual container 1 is, for example, 150 ml or more and 600 ml or less. In the example shown, a size of the dual container 1 in the bottle axis O direction is about 150 mm, and an inner capacity of the dual container 1 is 200 ml.

The mouth portion 13 of the dual container 1 is formed in a cylindrical shape extending upward from an upper end portion of the shoulder portion 15.

A flange portion protruding outward in the radial direction and extending continuously throughout the entire length in the circumferential direction is formed on an upper end portion of the mouth portion of the inner container 12. The flange portion is placed on an upper end opening edge of the mouth portion of the outer container 11.

A locking protrusion 18 to which a cap (that is not shown) is locked, a sealed protrusion 19 onto which an outer circumferential portion of the cap (that is not shown) is fitted, and a neck ring 20 are formed on an outer circumferential surface of the mouth portion of the outer container 11 in sequence from top to bottom.

The locking protrusion 18, the sealed protrusion 19, and the neck ring 20 protrude outward from the mouth portion of the outer container 11 in the radial direction, and extend continuously throughout the entire length in the circumferential direction. A gap (i.e., a space) between the outer circumferential surface of the sealed protrusion 19 and the inner circumferential surface of the outer circumferential portion of the cap (that is not shown) is air-tightly sealed. An outer diameter of the neck ring 20 is greater than an outer diameter of the sealed protrusion 19.

Further, the cap may be threaded onto the mouth portion of the outer container 11. The neck ring 20 may not be provided.

An outside air introduction hole 17 configured to introduce outside air into a space with the inner container 12 is provided above the shoulder portion 15 according to the reduction in contents. In the example shown, the outside air introduction hole 17 is formed in the mouth portion of the outer container 11. The outside air introduction hole 17 are located on the outermost side of the sealed protrusion 19 in the radial direction, and located above a sealing surface continuously extending throughout the entire length in the circumferential direction.

Further, a forming position of the outside air introduction hole 17 is not limited to the mouth portion of the outer container 11. A forming position of the outside air introduction hole 17 may be, for example, the drum portion, the shoulder portion, or the bottom portion, other than the mouth portion, of the outer container 11, and may be a space between the upper end opening edge of the mouth portion of the outer container 11 and the lower surface of the flange portion of the mouth portion of the inner container 12, or the like.

The shoulder portion 15 extends further outward in the radial direction as it goes downward from the lower end portion of the mouth portion 13.

At least a lower portion of the drum portion 16 extends outward in the radial direction as it goes downward. In the example shown, an upper portion of the drum portion 16 extends almost straight in the bottle axis O direction. A portion of the lower portion of the drum portion 16 except a lower end portion 16 a exhibits a curved shape that is recessed inward in the radial direction when seen from an outer side in the radial direction. The lower end portion 16 a of the drum portion 16 exhibits a curved shape protruding outward in the radial direction when seen from an outer side in the radial direction. An outer circumferential surface of the lower end portion 16 a of the drum portion 16 is a maximum outer diameter portion having a largest outer diameter in the dual container 1. The outer diameter of the maximum outer diameter portion is, for example, 58 mm or more and 74 mm or less. In the example shown, an outer diameter of the maximum outer diameter portion of the dual container 1 is about 62 mm. Further, the drum portion 16 may extend outward in the radial direction as it goes downward from above throughout the entire length in the bottle axis O direction. Among the lower portion of the drum portion 16, the lower end portion 16 a of the drum portion 16 and the lower portion of the drum portion 16, the portions except the lower end portion 16 a may extend in a linear shape or may extend in a curved shape when seen from an outer side in the radial direction.

The bottom portion 14 is formed in a cylindrical shape with a bottom. An outer circumferential portion 14 b of the bottom portion 14 extends inward in the radial direction as it goes downward. The bottom portion 14 exhibits a circular shape through the entire length in the bottle axis O direction when seen in a lateral cross-sectional view perpendicular to the bottle axis O.

At least the drum portion of the outer container 11 is able to be squeezed (i.e., elastically deformed), and the inner container 12 is deflated according to the squeeze deformation of the outer container 11. The drum portion 16, the shoulder portion 15 and the bottom portion 14 are continuous with each other in the bottle axis O direction with no step difference.

The drum portion 16 and the shoulder portion 15 are configured as a plurality of surface portions 31 are continuous with each other via corner portions 32 in the circumferential direction. In the example shown, the drum portion 16 and the shoulder portion 15 exhibit a regular hexagonal shape when seen in a lateral cross-sectional view. Further, the shoulder portion 15 may exhibit, for example, a circular shape, an elliptical shape, or the like, when seen in a lateral cross-sectional view.

The surface portions 31 exhibit a curved shape protruding outward in the radial direction when seen in a lateral cross-sectional view perpendicular to the bottle axis O. Further, the surface portions 31 may exhibit a linear shape when seen in a lateral cross-sectional view.

The corner portions 32 are formed in a ridge shape extending in the bottle axis O direction when seen from an outer side in the radial direction. Further, the corner portions 32 may be formed in a belt shape having a width in the circumferential direction and extending in the bottle axis O direction when seen from an outer side in the radial direction.

Each of the corner portions 32 has a first longitudinal convex rib 33 extending in the bottle axis O direction when seen from an outer side in the radial direction. Further, the first longitudinal convex rib 33 may not be provided.

The first longitudinal convex rib 33 is separately formed in each of the plurality of corner portions 32. The first longitudinal convex rib 33 extends straight in the bottle axis O direction when seen from an outer side in the radial direction. A width of the first longitudinal convex rib 33 is greater than a width of the corner portion 32. The first longitudinal convex rib 33 is formed integrally from a connecting portion (hereinafter, referred to as a lower end portion) 13 a of the mouth portion 13 to the shoulder portion 15 to a central portion of the drum portion 16 in the bottle axis O direction via the shoulder portion 15. That is, an upper end portion of f the first longitudinal convex rib 33 is located at the lower end portion 13 a of the mouth portion 13, and a lower end portion of the first longitudinal convex rib 33 is located at a central portion of the drum portion 16 in the bottle axis O direction.

Here, the lower end portion 13 a of the mouth portion 13 is a boundary portion located below the neck ring 20 and extending during blow molding. An upper gap is provided between the outer container 11 and the inner container 12 in the lower end portion 13 a of the mouth portion 13. The upper gap is provided intermittently throughout the entire length in the circumferential direction. The upper gap is provided even in a state before the dual container 1 becomes a biaxial stretching blow container, i.e., a state in which the inner preform configured to form the inner container 12 is fitted into the outer preform configured to form the outer container 11. Further, a configuration in which a plurality of longitudinal ribs are formed on an outer circumferential surface of an upper portion of the inner preform at intervals in the circumferential direction, and an upper gap is provided intermittently by the plurality of longitudinal ribs throughout the entire length in the circumferential direction may be employed. The upper gap may extend continuously throughout the entire length in the circumferential direction.

In the first longitudinal convex rib 33, a protrusion height of the portion located between both end portions in the bottle axis O direction is equal to the entire length in the bottle axis O direction, and a protrusion height of both end portions in the bottle axis O direction is decreased outward in the bottle axis O direction (i.e., an outer side of the dual container 1 in the bottle axis O direction). In the first longitudinal convex rib 33, a width of the portion located between both end portions in the bottle axis O direction is equal to the entire length in the bottle axis O direction, and a width of both end portions in the bottle axis O direction is decreased outward in the bottle axis O direction (i.e., an outer side of the dual container 1 in the bottle axis O direction). The first longitudinal convex rib 33 exhibits a convex curved shape when seen in a lateral cross-sectional view.

In the shoulder portion 15, a longitudinal groove 34 extending in the bottle axis O direction when seen from an outer side in the radial direction is formed in the portion located between the first longitudinal convex ribs 33 neighboring in the circumferential direction. Further, the longitudinal groove 34 may not be provided.

The longitudinal groove 34 extends straight in the bottle axis O direction when seen from an outer side in the radial direction. An upper end portion of the longitudinal groove 34 is located below the lower end portion 13 a of the mouth portion 13. The longitudinal groove 34 is formed integrally from the upper end portion of the shoulder portion 15 to the upper end portion of the drum portion 16. Further, the longitudinal groove 34 may be formed only in the shoulder portion 15. A width of the longitudinal groove 34 is greater than a width of the first longitudinal convex rib 33. Further, the width of the longitudinal groove 34 may be smaller than the width of the first longitudinal convex rib 33.

In the longitudinal groove 34, a depth of the portion located between both end portions in the bottle axis O direction is equal to the entire length in the bottle axis O direction, and a depth of both end portions in the bottle axis O direction becomes shallower outward in the bottle axis O direction (i.e., an outer side of the dual container 1 in the bottle axis O direction). In the longitudinal groove 34, a width of the portion located between both end portions in the bottle axis O direction is equal to the entire length in the bottle axis O direction, and a width of both end portions in the bottle axis O direction becomes narrower outward in the bottle axis O direction (i.e., an outer side of the dual container 1 in the bottle axis O direction). The plurality of longitudinal grooves 34 are formed in the surface portions 31 at intervals in the circumferential direction. The one longitudinal groove 34 is formed in a central portion of the surface portion 31 in the circumferential direction. The inner surface of the longitudinal groove 34 exhibits a concave curved shape the when seen in a lateral cross-sectional view.

A second longitudinal convex rib 35 extending in the bottle axis O direction when seen from an outer side in the radial direction is formed between the first longitudinal convex ribs 33 neighboring in the circumferential direction. Further, the second longitudinal convex rib 35 may not be provided.

The second longitudinal convex rib 35 extends straight in the bottle axis O direction when seen from an outer side in the radial direction. The second longitudinal convex rib 35 is formed integrally with the shoulder portion 15 and the drum portion 16. A length of the second longitudinal convex rib 35 is smaller than a length of the first longitudinal convex rib 33. A lower end portion of the second longitudinal convex rib 35 is located below the lower end portion of the longitudinal groove 34 and above the lower end portion of first longitudinal convex rib 33. Positions of upper end portions of the second longitudinal convex rib 35 and the first longitudinal convex rib 33 in the bottle axis O direction are the same as each other.

A protrusion height of the second longitudinal convex rib 35 is equal to a protrusion height of the first longitudinal convex rib 33. Further, the protrusion heights of the second longitudinal convex rib 35 and the first longitudinal convex rib 33 may be different from each other. A width of the second longitudinal convex rib 35 is smaller than a width of the first longitudinal convex rib 33. Further, the width of the second longitudinal convex rib 35 may be greater than the width of the first longitudinal convex rib 33.

In the second longitudinal convex rib 35, a protrusion height of the portion located between both end portions in the bottle axis O direction is equal to the entire length in the bottle axis O direction, and a protrusion height of both end portions in the bottle axis O direction is decreased outward in the bottle axis O direction (i.e., an outer side of the dual container 1 in the bottle axis O direction). In the second longitudinal convex rib 35, a width of the portion between both end portions in the bottle axis O direction is equal to the entire length in the bottle axis O direction, and a width of both end portions in the bottle axis O direction is narrowed outward in the bottle axis O direction (i.e., an outer side of the dual container 1 in the bottle axis O direction). The second longitudinal convex rib 35 exhibits a convex curved shape when seen in a lateral cross-sectional view.

The longitudinal groove 34 and the second longitudinal convex rib 35 are provided on the surface portions 31 alternately in the circumferential direction. At least one of the plurality of longitudinal grooves 34 provided in the surface portion 31 is provided between the first longitudinal convex rib 33 and the second longitudinal convex rib 35 neighboring in the circumferential direction. The first longitudinal convex rib 33, the longitudinal groove 34 and the second longitudinal convex rib 35 are provided continuously in the circumferential direction.

Further, the first longitudinal convex rib 33 and the second longitudinal convex rib 35 may be adjacent to each other in the circumferential direction with no interposition of the longitudinal groove 34. The first longitudinal convex rib 33, the longitudinal groove 34, and the second longitudinal convex rib 35 may be provided at intervals in the circumferential direction.

Here, in the embodiment, a circumferential groove 21 recessed inward in the radial direction and extending in the circumferential direction is formed in the drum portion 16. An inner surface of the circumferential groove 21 exhibits a curved shape recessed inward in the radial direction when seen in a longitudinal cross-sectional view in the bottle axis O direction. A longitudinal convex portion 22 configured to disconnect the circumferential groove 21 in the circumferential direction is formed on the inner surface of the circumferential groove 21. The plurality of longitudinal convex portions 22 are provided at intervals in the circumferential direction. The plurality of longitudinal convex portions 22 are provided in the circumferential groove 21 at intervals throughout the entire length in the circumferential direction. The longitudinal convex portion 22 is formed in a rectangular parallelepiped shape elongated in the bottle axis O direction.

In the example shown, the circumferential groove 21 is provided in a lower portion of the drum portion 16. The circumferential groove 21 is provided in the lower end portion 16 a of the drum portion 16. The circumferential groove 21 is provided in the lower end portion 16 a of the drum portion 16 throughout the entire length in the bottle axis O direction.

The longitudinal convex portion 22 is defined by an outer surface facing outward in the radial direction, and a pair of side surfaces that connect both end portions of the outer surface in the circumferential direction to an inner surface of the circumferential groove 21. Side surfaces of the longitudinal convex portion 22 extend in a direction in which they are separated from each other in the circumferential direction as they go inward in the radial direction. The outer surface of the longitudinal convex portion 22 is continuous with an outer circumferential surface of each portion continuous with the circumferential groove 21 of the dual container 1 in the bottle axis O direction with no step difference in the bottle axis O direction. The upper end portion of the outer surface of the longitudinal convex portion 22 is continuous with the outer circumferential surface of the drum portion 16 with no step difference, and the lower end portion of the outer surface of the longitudinal convex portion 22 is continuous with the outer circumferential surface of the outer circumferential portion 14 b of the bottom portion 14 with no step difference.

As described above, according to the dual container 1 of the embodiment, the circumferential groove 21 is formed in the drum portion 16. Accordingly, rigidity of the drum portion of the outer container 11 can be increased, and after the drum portion 16 is squeezed inward in the radial direction by applying an external force to the drum portion 16, the drum portion of the outer container 11 can be smoothly restored when the external force (i.e., pressing force) is released.

The longitudinal convex portion 22 configured to disconnect the circumferential groove 21 in the circumferential direction is formed on the inner surface of the circumferential groove 21. Accordingly, the outside air introduced between the inner container 12 and the outer container 11 from the outside air introduction hole 17 can easily pass through the longitudinal convex portion 22 in the bottle axis O direction, and the inner container 12 can easily peel off from the inner surface of the outer container 11 in the circumferential groove 21 from the longitudinal convex portion 22 as an origin. Accordingly, the inner container 12 including the portion where the circumferential groove 21 is provided can smoothly peel off from the inner surface of the outer container 11 according to reduction of the accommodated contents.

An outer surface of the longitudinal convex portion 22 facing outward in the radial direction is continuous with the outer circumferential surface of each portion of the circumferential groove 21 of the dual container 1 in the bottle axis O direction with no step difference in the bottle axis O direction. Accordingly, the outside air introduced between the inner container 12 and the outer container 11 from the outside air introduction hole 17 can reliably easily pass through the longitudinal convex portion 22 in the bottle axis O direction.

At least the lower portion of the drum portion 16 extends outward in the radial direction as it goes downward, and the circumferential groove 21 is provided in the lower portion where the outer container 11 is hard to be restored after squeeze deformation. Accordingly, rigidity of the lower portion of the drum portion of the outer container 11 can be increased, and the lower portion of the drum portion of the outer container 11 can be easily restored after squeeze deformation.

Since the longitudinal convex portion 22 is formed on the inner surface of the circumferential groove 21, smooth peeling of the inner container 12 from the inner surface of the outer container 11 in the lower portion of the drum portion 16 can be secured.

Further, the technical scope of the present invention is not limited to the above-mentioned embodiment, and various modifications may be made without departing from the spirit of the present invention.

A step portion may be provided between the outer surface of the longitudinal convex portion 22 and the outer circumferential surface of each portion continuous with the circumferential groove 21 of the dual container 1 in the bottle axis O direction.

The longitudinal convex portion 22 may be formed in, for example, a cube shape or the like.

The drum portion 16 may extend straight in the bottle axis O direction throughout the entire length in the bottle axis O direction.

In addition, the components of the embodiment may be appropriately replaced with known components and the embodiment and the variants may be appropriately combined without departing from the spirit of the present invention.

While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. 

What is claimed is:
 1. A dual container comprising: an inner container deformed to reduce a volume according to reduction of accommodated contents; and an outer container in which the inner container is inserted and which is elastically deformable, wherein a mouth portion, a shoulder portion, a drum portion, and a bottom portion are arranged in sequence from top to bottom in a bottle axis direction, an outside air introduction hole configured to introduce outside air between the outer container and the inner container according to reduction of contents is provided, a circumferential groove recessed inward in a radial direction and extending in a circumferential direction is formed in the drum portion, and a longitudinal convex portion configured to disconnect the circumferential groove in the circumferential direction is provided on an inner surface of the circumferential groove.
 2. The dual container according to claim 1, wherein an outer surface of the longitudinal convex portion, the outer surface facing outward in the radial direction, is continuous with an outer circumferential surface of each portion of the dual container, the portion being continuous with the circumferential groove in the upward/downward direction, with no step difference in the upward/downward direction.
 3. The dual container according to claim 1, wherein at least a lower portion of the drum portion extends outward in the radial direction as it goes downward, and the circumferential groove is provided in the lower portion of the drum portion.
 4. The dual container according to claim 2, wherein at least a lower portion of the drum portion extends outward in the radial direction as it goes downward, and the circumferential groove is provided in the lower portion of the drum portion. 